This specification relates to controlling of termination resistance values in memory modules.
A typical memory system includes memory modules that are arranged in slots. Each memory module includes a number of memory chips. For example, the memory module can be a dual inline memory module (DIMM) and the memory chips can be dynamic random access memory chips (DRAMs). Memory modules are physically placed in slot connectors and are electrically coupled to other components, e.g., one or more memory controllers, through channels and buses. These channels and buses form transmission lines that are electrically terminated at the connected DIMMs. A memory controller can select any of the DIMMs in a channel for reading or writing, but it will only access one DIMM at a time. The slot in which the DIMM accessed for reading or writing is located is referred to as the “active” slot, while slots in which the other non-accessed DIMMs are located are referred to as the “standby” slots.
A typical DIMM can have a single rank or multiple ranks. A rank is an independent set of DRAMs within the DIMM that can be simultaneously accessed for the full data bit-width of the DIMM, such as 72 bits. The rank to which data is being written is called the target rank for writes. The rank from which data is being read is called the target rank for reads.